Celtics point guard Isaiah Thomas reacts to a foul call during Game 2. (Maddie Meyer/Getty Images)

Celtics point guard Isaiah Thomas had one of the greatest performances in NBA playoff history in Tuesday’s 129-119 win over the Wizards, scoring 29 of his 53 points in the fourth quarter and overtime on his late sister’s 23rd birthday. Thomas shot 18 for 33 from the field and 12 for 13 from the free throw line, where a few sharp-eyed rules sticklers noticed he committed a violation that’s rarely called on almost every one of his attempts.

Thomas, who made 91 percent of his free throws during the regular season, has an unorthodox shooting action at the line. Like most shooters, he transfers his weight from his heels to his toes as he releases the ball. Rather than returning his weight to his heels after he follows through, Thomas remains on his toes and often steps into the lane with his left foot before the ball reaches the cylinder. Here’s a screenshot from Thomas’s game-tying free throw with 14 seconds left in regulation on Tuesday:

Isaiah Thomas’s left foot lands in the lane before the ball reaches the basket. (Via TNT)

The free throw shooter may not step over the plane of the free throw line until the ball touches the basket ring, backboard or the free throw ends.

Thomas’s illegal free throw shooting form has been the subject of multipleRedditthreads this season, including one titled, “Why are we ignoring how Isaiah Thomas cheats on every free throw?” Tweets about Thomas stepping over the line, which he does on most, but not all, of his attempts, seem to have increased in frequency during the playoffs.

So what’s the point of having a rule that isn’t enforced, especially one that’s more cut and dried than the countless moving screens and carrying violations that go uncalled during a typical NBA game?

“Sometimes rules are put in for intent purposes,” Steve Javie, who spent 25 years as an NBA referee and now serves an officiating analyst for ESPN, told The Post on Wednesday. “In this case, the rule prevents someone from shooting, on an intentional miss, and running and getting the rebound first. It’s one of these rules where as an official you’re thinking, what advantage did the guy really gain from it? He shoots it from behind the line and his foot steps into the lane. Well, to me, he didn’t gain an advantage. But if he shoots the ball and then runs and grabs the rebound, it’s an obvious advantage.”

Another reason the rule exists is to prevent players from dunking free throws. College basketball introduced a rule in 1956 that prohibited players from having a foot across the free throw line until the ball hit the cylinder or the backboard in response to 7-foot-1 Wilt Chamberlain, who would get a running start, jump behind the free throw line and then slam the ball through the hoop. The 5-foot-9 Thomas isn’t a threat to pull that off.

NBA referees seldom call lane violations on free throw shooters, except in cases where the shooter gains a clear advantage, but it wouldn’t be unprecedented if they decided to crack down on Thomas’s habitual line-stepping.

“I was wondering when someone was going to bring this up, because a few years back when I was refereeing, Shaquille O’Neal got in the habit of doing it,” Javie said. “He would shoot it, get off balance and just like Isaiah Thomas is doing, his foot would go over the line and the ball would go in.”

Or, in O’Neal’s case, clang off the rim. In 2001, Dallas Mavericks owner Mark Cuban was among those who complained to the league about O’Neal stepping over the line on his free throws. NBA Senior Vice President Stu Jackson responded by telling referees, including Javie, to keep a closer eye on O’Neal at the foul line.

“Mark, you make too much money to be whining and crying,” O’Neal told the Los Angeles Daily News. “Let me tell you something — crying is not going to stop Shaq O’Neal. It’s just going to get me angry. And when I’m angry, you won’t like me when I’m angry.”

The Wizards don’t want to see an angry Thomas, and to be clear, nobody from the organization, to my knowledge, is whining and crying to the league about his illegal free throw shooting form. Thomas’s occasional baby step into the lane is not the reason the Wizards are trailing two games to none heading into Thursday’s Game 3 at Verizon Center, and frankly, Thomas probably could’ve shot 12 for 13 from the line blindfolded and with his feet duct-taped to the floor on Tuesday.

“To me, it doesn’t help him, but is it a violation? Yeah, it is,” Javie said. “I don’t know what’s going to happen going forward. I’d be curious to see the next game. The league may react to it and tell the officials. We’ll see what happens. Once you call it a few times, players do adjust and they stop.”

Isaiah Thomas on but he does a lane violation on every single free throw!